The invention relates to an optical system for a bar code reader, and in particular to an improved sensor array and illumination method adapted for use with a bar code reader optical system using a modified Scheimpflug arrangement.
The optical system of a bar code scanner comprises apparatus for illuminating the bar code tag and apparatus for detecting the reflected light from the bar code tag. An optical system images the light reflected from the bar code tag onto a detector which produces an electronic signal that represents the light and dark areas of the bar code.
An ideal bar code scanner would have a large working range, while being capable of reading bar codes with very narrow bars and spaces. However, the optical requirements for these two goals conflict. Maximizing the working range requires a small lens aperture (a large f/#), while maximizing resolution requires a large lens aperture because of diffraction effects.
One type of scanner optical system, referred to as a "detector resolved" optical system, forms an image of the bar code tag on the detector, which produces an electrical signal corresponding to the dark and light pattern of the bars and spaces in the bar code. The operation of this type of scanner is limited by the ability of its detector to resolve the optical image of the bar code into an electronic signal.
The detector may be a single element photodetector, or a multiple element photodetector array that spans the image of the bar code. In a single element system, an optical scanning mechanism, such as a rotating mirror, sweeps the image of the bar code across the detector element to produce the signal. This sweep produces a serial image of a "scan line", a line through the bar code perpendicular to the bars and spaces. In a multiple element system, the detector array is electronically scanned.
The working range of a typical detector resolved optical system is the range of distance from the lens over which the image of the bar code tag remains sufficiently focused for the detector array and processing electronics to recognize bars from spaces and to measure the widths of the bars and spaces accurately. The working range is a function of the aperture of the lens system (the f/#) and the resolution of the bar code being read.
One proposal to overcome many of the problems of conventional detector resolved optical systems is described in a copending application OPTICAL SYSTEM FOR A LARGE DEPTH-OF FIELD BAR CODE SCANNER, assigned in common with this application. The desired working range is imaged onto the detector array using a modified Scheimpflug arrangement, with the array tilted with respect to the optical axis of the lens. Thus, the working range is not determined by the lens aperture, but by the dimension and orientation of the detector array. The lens aperture can be as large as desired to maximize resolution and minimize illumination needs, and thus power consumption, without adversely affecting the working range. For a high resolution bar code tag, the practical working range is about 12.5 cm to 50 cm. This is about twice as large as the working range of the best scanners with conventional optical systems.
In one embodiment, a two dimensional grid detector array is imaged over the working range in one dimension and over the width of the bar code tag in the other dimension. The array is similar to the kind of arrays used in video applications, having a large number of pixels in both dimensions. The data output rate is about 4,000,000 pixels per second, with most of the data discarded. Video signal processing techniques are used to reconstruct the in-focus image of a section of the bar code, and to produce a suitable output signal.
The object of the present invention is to provide an improved sensor array and illumination scheme for a bar code scanner using a modified Scheimpflug arrangement optical system that has simplified signal processing.
This and other advantages are realized by the preferred embodiment of the invention which includes a photodetector comprising a multi-element array of photosensors positioned in one Scheimpflug plane and an illumination beam in the form of a sheet of light along the other Scheimpflug plane. The elements of the detector array have a long axis and a short axis, with the long axis of the elements arranged substantially perpendicular to the image of the illuminated strip of the bar code tag. The projection of the long axis of the elements of the array onto the other Scheimpflug plane defines the working range of the optical system. So long as the bar code tag intersects the illumination beam within the working range, the image of the illuminated strip of the bar code tag will be substantially in focus across the elements of the detector array. Filtering allows substantially only the light from the illuminated strip to reach the detector array. The output signal from the array can be processed with conventional digitizing and decoding circuitry with audio signal processing techniques.
An important aspect of the invention is that substantially all of the light that reaches the detector array is useful signal, and all the output data from the array during a bar code scan is relevant. The cooperation of the illumination system, the Scheimpflug optics and the bandpass filter makes this possible. Referring to FIG. 1, the sheet of light from the illumination section 113 illuminates only a thin strip across the bar code tag. The modified Scheimpflug optical arrangement guarantees that this illuminated strip will form a focused image on the detector array 131. The bandpass filter 137 rejects substantially all other light. As a result, substantially all of the light that reaches the detector array 131 is useful signal.
The elements of detector array 131 are constructed to take advantage of this. The detector array of the present invention has significantly fewer elements than a two dimensional grid type array. This makes the array less expensive to build and results in lower power consumption.
The detector array also has several advantages from the signal processing point of view. All the output data from the array during a bar code scan is relevant, in contrast to the output data from a grid type array in which the data from the few lines of elements that receive a focused image must be selected from the data stream. The array data output rate is about 40,000 pixels per second to produce a scanning rate equivalent to that of bar code scanners currently available. This rate is 100 times lower than the grid type array data rate, so analog processing with audio techniques is adequate.